(1) Field of the Invention
The present invention relates to the field of assisting the piloting of vehicles, and in particular of rotary wing aircraft.
The present invention relates to a method and a device for determining and optimizing parameters that are characteristic of the operation of a vehicle, and in particular of a rotary wing aircraft.
(2) Description of Related Art
A vehicle is generally operated while monitoring several characteristic parameters by using instruments situated on an instrument panel of the vehicle. These characteristic parameters represent the current operation of the vehicle and in particular of its engine or its power plant.
For physical reasons, there exist numerous limits on such characteristic parameters that need to be taken into account at all times during the operation of the vehicle. These various limits may depend on external conditions and also on the mode of operation of the vehicle.
For example, for a self-propelled vehicle, these characteristic parameters may be the speed of rotation of its engine, the temperature of its engine cooling water, or the temperature of its engine lubricating oil.
In another example, the vehicle may be a rotary wing aircraft having a power plant with two turboshaft engines and a main gearbox, the power plant driving rotation of at least one main rotor and possibly of an anti-torque rotor, such as a tail rotor. Under such circumstances, these characteristic parameters may include, among others: the speed of rotation Ng of the gas generator of each engine, the gas ejection temperature T4 at the inlet to the free turbine of each engine, and the drive torque Cm from each engine.
Thus, while a vehicle is in operation, a pilot of the vehicle needs to monitor continuously the current values of these characteristic parameters on various instruments situated on the instrument panel of the vehicle, and to compare the current values of these characteristic parameters with their respective limits.
For a rotary wing aircraft having a power plant with two turboshaft engines, the pilot needs to act continuously during operation of the aircraft to monitor at least three instruments per engine, i.e. at least six instruments. The pilot is then assumed to be capable of detecting, for each engine, any inconsistency between the current values of those characteristic parameters and their respective limits on those at least six instruments. This requires particular attention on the part of the pilot, who also needs to be concentrating on actually flying the aircraft.
In addition, those limits generally differ depending on the stage of flight of the aircraft and/or on external conditions, such as altitude, for example. During each stage of flight and/or depending on external conditions and on the mode of operation of the power plant, the maximum power that can be delivered by the power plant differs, and the length of time for which it is available can also be limited.
For example, while the aircraft is taking off, a maximum takeoff power MTOP can be used for a limited duration of about five to ten minutes, corresponding to a level of torque for the main gearbox and to a level of power and of heating for each engine that can be accepted without significantly degrading the power plant. Likewise, after the stage of taking off, there is a maximum continuous power MCP that can be used continuously without any limit on duration.
Furthermore, there are also contingency ratings at extra high power that are used on a power plant having at least two engines when one of the engines has failed. The valid engine(s) can then deliver contingency power levels for limited durations, these contingency power levels being greater than the maximum continuous power MCP in order to compensate for the failure. Nevertheless, using such contingency ratings generally requires specific maintenance operations to be performed thereafter.
Consequently, the limits on the various parameters that are characteristic of the operation of the aircraft can differ, in particular depending on the power available from each engine.
At present, certain kinds of assistance given to the pilot make it possible to limit the parameters that the pilot must monitor.
In particular, Documents FR 2 749 545 and FR 2 756 256 are known, which describe a first limit indicator (FLI). That first limit indicator identifies among various characteristic parameters the characteristic parameter that is critical, i.e. the parameter that is the closest to its limit value. The current value of this critical characteristic parameter and its limit value are then grouped together on a single display, respectively for each engine, where appropriate, thereby limiting the number of instruments needed for monitoring the operation of the aircraft, thereby simplifying the pilot's task. Such FLIs thus make it possible to display an available power margin for the aircraft or for each engine, by means of the current value of the critical characteristic parameter and its limit value.
For example, the current value of the critical characteristic parameter and its limit value may be displayed on a scale graduated in engine torque for each of the engines, thereby characterizing the power margin available from each engine of the aircraft, as described in Document FR 2 749 545.
The current value of the critical characteristic parameter and its limit value can also be displayed on a scale that is graduated in collective pitch, where the collective pitch is the angle of incidence of the blades of the main rotor of the aircraft relative to the incident wind, and as induced by a collective pitch control for those blades, thereby characterizing the power margin that is available for the aircraft as a whole, as described in Document FR 2 756 256.
Also known are Documents WO 2006/081334 and US 2005/278084 that describe indicators for an aircraft that determine and display at least the value of the critical characteristic parameter.
The indicator of Document WO 2006/081334 also determines and displays a characteristic parameter of value that is important and that may correspond to a predetermined maneuver of the aircraft.
The indicator of Document US 2005/278084 also determines and displays a predictive value for the critical characteristic parameter, which predictive value is determined as a function of its current value and of its variation.
In addition, Document EP 2 505 502 describes a method and a device for assisting in the piloting of an aircraft having two rotors, e.g. a lift rotor and an anti-torque tail rotor. A power margin relative to a known maximum power from the engine of the aircraft is determined using that one of the characteristic parameters that has a value that is the closest to its operating limit. A limit curve for available power that takes this power margin into account is displayed to the pilot in the form of a diagram in a reference constituted by a first collective pitch of the first rotor and a second collective pitch of the second rotor. A pointer that is characterized by the current positions of the first and second collective pitches is also displayed. The pilot thus knows the additional power that is available from the first and second rotors.
In addition, a curve that takes account of an engine damage threshold can be displayed, which threshold corresponds to the maximum power that can be used without running the risk of damaging the engine. This usable maximum power is determined on the basis of the known maximum power of the engine of the aircraft as supplied by the manufacturer of the engine.
Finally, Document DE 2 970 3902 forms part of the technological background of the invention.
Nevertheless, the limits used by the instruments of the panel instrument and by the FLI in particular do not represent the real limits for each engine, but rather predetermined limits that correspond for example to a minimum guaranteed power level from an engine over its entire lifetime.
Specifically, the engine manufacturer performs calculations or carries out tests to establish available power curves for a turboshaft engine as a function in particular of the altitude of the aircraft and of the outside temperature, with this being done for each of the power ratings that can be used for each engine. Furthermore, the manufacturer determines the available power curves for different degrees of aging of each engine from a new engine to an engine that has come to the end of its lifetime.
Consequently, a minimum power level guaranteed over the entire lifetime of an engine is defined. The value of this guaranteed minimum power varies in particular as a function of the altitude of the aircraft and the outside temperature, and it corresponds to the power delivered by an aged engine, i.e. an engine that has come to the end of its maximum lifetime. Thus, any engine in normal operation, i.e. an engine that has not suffered a failure, can always deliver power that is higher and at worst that is not less than the minimum power that is guaranteed for its entire lifetime.
As a result, the instrument panel instruments and the FLI in particular that make use of limits corresponding to the guaranteed minimum power are favorable in terms of safety, since the pilot always has a level of power that is genuinely available from each engine that is generally greater than and at worst not less than the maximum power indicated by the control panel instruments or by the FLI.
In contrast, the use of each engine is not optimized, since the power used is the minimum guaranteed power and not the maximum power that is genuinely available. Specifically, each engine is under-used. The use of limits corresponding to the maximum power that is genuinely available would enable the performance of the aircraft to be improved, such as the total mass it can transport or the range it can cover, for example.
In addition, the instruments of the control panel of a vehicle and also the FLI in aircraft indicate the current values and the limits of characteristic parameters. As a result, when a pilot envisages carrying out a maneuver, the pilot must rely on experience and on the difference that can be seen between these current and limit values in order to estimate whether there is a sufficient power margin on the characteristic parameters for performing the maneuver.
Thereafter, the pilot will receive confirmation only while performing the maneuver that none of the characteristic parameters has exceeded its limits and that the maneuver can be carried out in complete safety.
Otherwise, and depending on the maneuver being performed, the pilot may interrupt the maneuver in order to return to a safe stage of flight, such that each characteristic parameter remains below its limits. This applies typically when the pilot of an aircraft begins a descent and it is possible to use the inertia of the aircraft and/or the total power available from the power plant to perform a maneuver to avoid an obstacle. Nevertheless, for certain maneuvers, going back to the previous situation is not possible once the maneuver has been started and that can lead to an accident, e.g. while landing and during the transition from no ground effect to the ground effect applying.
Such poor estimation by the pilot of the available margin, in particular in terms of power, lies behind numerous rotary wing aircraft accidents, in particular during stages of landing, while hovering, in particular close to the ground, and while taking off in a purely vertical mode.